Yarns to be used as pile in cut pile carpets are currently 2-ply twisted for aesthetics and heat set in the plied condition to retain their twist when cut and subjected to normal wear. The feed yarns can be either continuous filaments or spun yarns. In the most common plying method, (cable twisting), two yarns either continuous filament or spun yarns are twisted together resulting in a yarn having zero twist in each component yarn. However, such twist plying has low productivity for two reasons; first, it is a slow operation limited to about 35 ypm by centrifugal force considerations and second, it is discontinuous due to the need to replace yarn packages in the bucket of the plying equipment. A faster more economical and more flexible continuous operation to make such yarns is greatly desired.
Carpet staple spun yarns used to feed the plying process are typically produced by ring spinning or wrap spinning processes. In ring spinning, production speeds are limited by the flyer to approximately 35 to 40 ypm. In wrap spinning, production speeds are limited by the spindle to approximately 100 ypm. Both of these processes are discontinuous (reducing throughput) by the wound package (ring spinning) or spindle package (wrap spinning).
Each yarn from the processes have unique characteristics. The ring spun yarns have a minimum number of staple fibers per cross section (typically 80 to 100) and a minimum twist level (&gt;1 tpi) to develop proper yarn tenacity for processing. The wrap spun yarns have zero twist spun yarn core over wrapped by a light denier continuous filament yarn to achieve proper tenacity. The wrap spun yarn also requires a minimum number of staple fibers per cross section (80-100) for processability.
Staple spun yarns used for carpets should desirably have as much bulk as possible to hide the backing and resist crushing loads. This bulk is mainly contributed by crimp introduced into synthetic fibers by one of several processes. However, these staple spun yarns require a substantial amount of real twist to hold the fibers together and contribute the tenacity necessary to wind and unwind the yarn and to weave or tuft it into carpet backing. Such twist compresses the fibers laterally and reduces their bulk. Bulk is also contributed by the retraction and crimp memory displayed by such fibers during hot wet processing of the yarns or carpets during twist setting, scouring and dyeing, but such crimp recovery is also inhibited by a high degree of twist.
Furthermore, the tenacity of such staple yarns depends also on the number of fibers in a given cross section of the yarn and on their length. It is known that a given number of long fibers makes a stronger yarn than short fibers at a given twist level, but strength also depends on the number of fibers which contribute the necessary frictional forces between fibers.
Large diameter crimped fibers resist compression and retain bulk better than small fibers, but large fibers must be fewer in number to make a yarn of given total size. Thus, the number of such large fibers may be too few to give adequate strength at a given amount of twist and a certain fiber length, when staple yarns are formed by twisting in the conventional manner.
A technique developed in the 1960's called rotor spinning or open end spinning feeds staple fibers to the inside wall of a cup-shaped rotor operating at high speed where centrifugal force compacts the fibers into a consolidation groove, then the fibers are led inward toward the axis of rotation and are removed through an axial passage. The rotation of the rotor twists the yarn to a degree dependent on the revolutions of the rotor and the removal speed, higher removal speed giving lower real twist of the fibers for a given rotational speed. Rotational speed is limited partly by the strength of the rotor but in practice is more often limited by the ability of the twisted staple to bear the tension required to counteract centrifugal force while removing it from the rotor groove. A low degree of real twist may produce a yarn which is too weak to be removed when the false twist after the navel has disappeared.